The present invention relates to a method of producing a cordierite ceramic honeycomb having a cordierite as a main ingredient of a crystal phase.
Generally, a cordierite ceramic honeycomb structural body is obtained by the steps of preparing raw materials becoming cordierite and forming agents, adding the forming agents into the raw materials becoming cordierite, mixing the forming agents and the raw materials becoming cordierite to obtain a raw material batch, extruding the raw material batch to obtain a formed body, drying the formed body, and sintering the formed body after drying. In the cordierite ceramic honeycomb structural body mentioned above, as a method of obtaining a cordierite ceramic honeycomb having a low thermal expansion coefficient, there is disclosed a method, in Japanese Patent Publication No. 5-82343, such that a cordierite ceramic honeycomb having a porosity of 30-42%, a thermal expansion coefficient along A-axis of not larger than 0.3xc3x9710xe2x88x926/xc2x0 C. and a thermal expansion coefficient along B-axis of not larger than 0.5xc3x9710xe2x88x926/xc2x0 C. is obtained by using talc having average particle size of 5-100 xcexcm, alumina having an average particle size of not larger than 2 xcexcm and high-purity amorphous silica having an average particle size of not larger than 15 xcexcm. Moreover, there is disclosed a cordierite ceramic honeycomb, in Japanese Patent Publication No. 4-70053, having a porosity of not larger than 30%, a thermal expansion coefficient along A-axis of not larger than 0.8xc3x9710xe2x88x926/xc2x0 C. and a thermal expansion coefficient along B-axis of not larger than 1.0xc3x9710xe2x88x926/xc2x0 C.
Recently, it is highly required to produce a thin wall honeycomb whose rib thickness is not larger than 100 xcexcm. In this case, in order to carry a catalyst easily, it is preferred to set a porosity of the honeycomb to not less than 30%. In addition, in order to prevent a rib failure, it is necessary to exclude coarse particles having a particle size larger than a slit width of a die from raw materials. However, in the known techniques mentioned above, there arise following problems. That is to say, fine alumina having an average particle size of not larger than 2 xcexcm used as one of raw materials has an advantage such that it decreases a thermal expansion coefficient. On the other hand, since fine alumina mentioned above has a strong particle agglomerating property and it is difficult to perform a classification, it is not possible to eliminate coarse particles form fine alumina. Therefore, alumina coarse particles remaining in fine alumina block a slit of the die during a honeycomb forming step utilizing the die, and this causes a rib failure of the formed honeycomb. Moreover, since fine alumina has fine particles as mentioned above, there is a drawback such that fine alumina decreases a porosity of the cordierite ceramic honeycomb. Further, high-purity amorphous silica used as one of raw materials has an advantage such that it decreases a thermal expansion coefficient. On the other hand, there is a drawback such that high-purity amorphous silica decreases a porosity of the cordierite ceramic honeycomb as compared with quartz silica and it is expensive.
An object of the invention is to eliminate the drawbacks mentioned above and to provide a method of producing a cordierite ceramic honeycomb which can obtain a thin wall honeycomb structural body having no rib failure and low thermal expansion coefficient inexpensively.
According to the invention, a method of producing a cordierite ceramic honeycomb comprises the steps of: preparing raw materials becoming cordierite and forming agents; adding the forming agents into the raw materials becoming cordierite; mixing the forming agents and the raw materials to obtain a raw material batch; extruding the raw material batch to obtain a formed body; drying the formed body; and sintering the formed body after drying; so as to obtain a honeycomb structural body having a cordierite crystal phase as a main ingredient, wherein, at the sintering step, a temperature descending rate at least from a maximum temperature to 1300xc2x0 C. is not larger than 100xc2x0 C./hour.
In the present invention, since, at the sintering step, a temperature descending rate at least from a maximum temperature to 1300xc2x0 C. is not larger than 100xc2x0 C./hour, it is possible to obtain a cordierite ceramic honeycomb having no rib failure and low thermal expansion coefficient inexpensively.
In a preferred embodiment of the present invention, quartz is used in a raw material batch becoming cordierite and alumina having an average particle size larger than 2 xcexcm is used. In this case, it is possible to obtain a honeycomb structural body inexpensively as compared with a honeycomb structural body obtained according to the known producing method. The thus obtained cordierite ceramic honeycomb shows a property such that a thermal expansion coefficient along A-axis is not larger than 0.4xc3x9710xe2x88x926/xc2x0 C. and a thermal expansion coefficient along B-axis is not larger than 0.6xc3x9710xe2x88x926/xc2x0 C. in a temperature range between 40xc2x0 C. and 800xc2x0 C. In the preferred embodiment, the thus obtained cordierite ceramic honeycomb shows a property such that a thermal expansion coefficient along A-axis is not larger than 0.3xc3x9710xe2x88x926/xc2x0 C. and a thermal expansion coefficient along B-axis is not larger than 0.5xc3x9710xe2x88x926/xc2x0 C. Further, a porosity of the cordierite ceramic honeycomb is larger than 30%. In addition, when lauric acid potash soap is used as a forming agent, when a temperature descending rate from a maximum temperature to 1250xc2x0 C. is not larger than 50xc2x0 C./hour, or, when a temperature maintaining time at a maximum material is not less than 6 hours, the present invention can be achieved more preferably.